Frame Sizing

I need help deciding as to what frame to buy. I want to start riding 20'', but I look through my DansComp mag and see many bikes from the same company, most with different geo in the frame. What is optimal for someone my size.. 5'7'' and around 145lbs?
I dont have the option of trying different bikes out, and will need to order my frame online.
What is going to give me the best feel, and what should I be careful of?

Thanks!

Edit: I would like something light, and I dont mind spending a bit more on a nice set up, as I plan on growing in skill with it.